FIG. 1 illustrates a tablet computer 100, which serves as an example pen-based electronic device. Tablet computer 100 comprises a writing surface 101, which serves as in input device when used in conjunction with pen 102. Tablet computer 100 can sense the location of touches of digitizing pen 102 on writing surface 101. Writing surface 101 may be touch sensitive, or other sensing means may be used.
One way the touch location information may be used is to enable entering of graphical information into tablet computer 100. For example, when programmed with appropriate software, tablet computer can record touches of digitizing pen 102 on writing surface 101, interpreting them as instructions for creating a drawing, and nearly simultaneously display a resulting drawing 103 on a display screen, which may be coincident with writing surface 101. Such a device may be called a digitizer.
In order to more naturally emulate traditional pencil-and-paper drawing, the system can also sense the force with which a user presses digitizing pen 102 against writing surface 101. For example, digitizing pen 102 may contain means for measuring the writing force, and may communicate the force information to computer 100 by radio signals or other means. Computer 100 may incorporate the force information into its drawing display, for example by displaying a wider line when the pen force is high than when the pen force is low. A user can then make bold drawing strokes by pressing digitizing pen 102 firmly against writing surface 101, and lighter drawing strokes by pressing less firmly, just as the user would do when using a pencil and paper.
Component and manufacturing variations can cause one pen to perform differently than another pen, with the same computer 100. For example, the force sensing element of one pen, often called a “tip switch”, may require a writing force of 15 grams before it signals that the pen and writing surface 101 have come in contact, while another pen may not register contact until the writing force is 20 grams. The two pens may also give different readings at other forces as well, causing the system to produce different results. For example, one pen may produce a very bold line when pressed against writing surface 101 with a force of 75 grams, while another pen may produce a narrow line when pressed against writing surface 101 with the same force.
It is desirable that different pens and devices perform consistently so that a particular device produces similar results for a particular user regardless of which pen is used. Inconsistent results may arise with other pen-enabled electronic devices as well, such as personal digital assistants, pen-enabled notebook computers, or other devices.